Aug 5, 2012

In addition to their
role in determining sex, the sex chromosomes, especially X chromosomes, have
genes for many characters unrelated to sex. A gene located on either sex
chromosome is called a sex–linked
gene, although in humans the term has historically referred
specifically to a gene on the X chromosome. (Note the distinction between the
terms sex–linked gene, referring to a gene on a sex chromosome, and linked
genes, referring to genes on the same chromosome that tend to be inherited
together.) Sex–linked genes in humans follow the same pattern of inheritance
that Morgan observed for the eye–color locus in Drosophila.
Fathers pass sex–linked alleles to all of their daughters but to none of their
sons. In contrast, mothers can pass sex–linked alleles to both sons and
daughters.

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The
transmission of sex–linked recessive traits. In this diagram, the superscript A represents a dominant allele carried
on the X chromosome, and the superscript a represents a recessive allele.
Imagine that this recessive allele is a mutation that causes a sex–linked
disorder, such as color blindness. White boxes indicate unaffected individuals,
light–coloured boxes indicate carriers, and dark–coloured boxes indicate
individuals with the sex–linked disorder

If a sex–linked trait
is due to a recessive allele, a female will express the phenotype only if she
is a homozygote. Because males have only one locus, the terms homozygous and
heterozygous lack meaning for describing their sex–linked genes (the term
hemizygous is used in such cases). Any male receiving the recessive allele from
his mother will express the trait. For this reason, far more males than females
have sex–linked recessive disorders. However, even though the chance of a
female inheriting a double dose of the mutant allele is much less than the
probability of a male inheriting a single dose, there are females with
sex–linked disorders. For instance, colour blindness is a mild disorder
inherited as a sex–linked trait. A colour–blind daughter may be born to a
colour–blind father whose mate is a carrier. However,
because the sex–linked allele for colour blindness is relatively rare, the
probability that such a man and woman will mate is low.

A number of human sex–linked disorders are much more serious
than colour blindness. An example is Duchenne muscular dystrophy, which affects about
one out of every 3,500 males born in the United States. The disease is
characterised by a progressive weakening of the muscles and loss of
coordination. Affected individuals rarely live past their early 20s.
Researchers have traced the disorder to the absence of a key muscle protein
called dystrophin and have mapped the gene for this protein to a specific locus
on the X chromosome.

Haemophiliais a sex–linked recessive disorder defined by the absence of one or more of the
proteins required for blood clotting. When a person with haemophilia is injured,
bleeding is prolonged because a firm clot is slow to form. Small cuts in the
skin are usually not a problem, but bleeding in the muscles or joints can be
painful and can lead to serious damage. Today, people with haemophilia are
treated as needed with intravenous injections of the missing protein.